I'm trying to create a count of how many hearts I collect and a NullReferenceException appears each time?

I'm a beginner on Unity and just learning from my lecturer's notes. But I'm trying to create an remove object script so that when my player hits an object(in my case, a heart) It will disappear and it will appear on my Interface, which say Items Picked Up. My Objects can be collected but I can't get the name Heart to appear and when I do collide with the object, this error appears : ( with removeObjectScript being the name)

NullReferenceException: Object reference not set to an instance of an object

removeObjectScript.OnTriggerEnter (UnityEngine.Collider other)

Due to fact that I have no past knowledge of Unity, I really don't know what it means, in regards of my script. This is the code

 private var playerData : GameObject;
 private var playerDataScript : PlayerDataScript;
 
 private var pickups = new Array();
 private var itemsPickedUp : int;
 
 
 function OnTriggerEnter (other : Collider) {
     Debug.Log("Collision");
     if(other.name == "Player")
     {
      
       //Debug.Log(other name);
       pickups = getPickups();
       pickups.Add("Heart");
       setPickups(pickups);
       itemsPickedUp = getNoOfPickups();
       itemsPickedUp++;
       setNoOfPickups(itemsPickedUp);
       Destroy(gameObject);
       }
       
 }
 
 function getPickups() : Array{
      playerData = GameObject.Find("PlayerData");
      if(playerData != null)
      {
         playerDataScript = playerData.GetComponent(PlayerDataScript);
         return playerDataScript.playerPickup;
         }
 }
 
 function setPickups(pickupsArray : Array){
      playerData = GameObject.Find("PlayerData");
      if(playerData != null)
      {
         playerDataScript = playerData.GetComponent(PlayerDataScript);
        playerDataScript.playerPickup = pickupsArray;
        
         }
 }
 function getNoOfPickups() : int{
      playerData = GameObject.Find("PlayerData");
      if(playerData != null)
      {
         playerDataScript = playerData.GetComponent(PlayerDataScript);
         return playerDataScript.playerItemsPickedUp;
         }
 }
 function setNoOfPickups(numPickups : int){
      playerData = GameObject.Find("PlayerData");
      if(playerData != null)
      {
         playerDataScript = playerData.GetComponent(PlayerDataScript);
          playerDataScript.playerItemsPickedUp = numPickups;
         }
 }

Am I missing something on my other scripts, because I've looked at other help and I still have no idea what it means to me as this is the first time I've done anything with programming.